'Ask me what that is," I say to my girlfriend, nodding at my new purchase, a small tree in a pot on the patio. I've been hoping she'd ask me of her own accord, but it's been a few days and she hasn't, almost certainly on purpose – to hurt my feelings. "Go on then, what is it?" she sighs.

"Ah ha," I say, whipping out the magnifying glass I just happen to have about my person. I make a play of examining the evidence, the dark green shiny leaves, the smell that comes off when I crush one in my fingers. Then I stand up triumphantly and declare: "A lemon tree, my dear Watson."

"Vicky!" she says. "My name is Vicky. Who the frig is Watson? Have you been cheating on me?"

So that didn't work. Puns never do. They're never funny, always laboured, and yet sometimes you find yourself being drawn irrevocably towards them. I was pleased that Mark Gatiss, who wrote this final episode of Sherlock (BBC1, Sunday), also fell into the trap.

"Meretricious," says Watson. "And a happy new year," snaps Holmes, quick as you like. OK, so less laboured than mine, but still a pun. Yes, it's Sherlock again, I'm afraid. Well, it really is the only thing on at the moment. And it is rather fabulous.

Moriarty, his arch enemy, turns out to be this Graham Nortonesquecharacter. Ooh, Sherlock, I'm so going to kill you. But the Reichenbach Falls have been replaced by a London swimming pool. There are nods to Sir Arthur Conan Doyle's stories all over the place – nice to spot, but no problem if you can't. There's a bit of The Final Problem and A Scandal in Bohemia, a hint of The Five Orange Pips, almost all of The Bruce-Partington Plans, and no doubt loads more I missed.

Of course, it's all been shunted forward 120 years or so, so The B-P Plans concern a new missile-defence system, and the pips are not the sort that may one day turn into an orange tree (my dear Watson), but the kind you get on the speaking clock. It may sound like a big dog's dinner (the Hound of the Baskervilles's, perhaps), but no, it's all been beautifully borrowed and remixed, a mash-up that totally works. It's an edge-of-the seat ride, as well – Conan Doyle for the CSI generation.

My job is to nit-pick, though. And I've (possibly) found a few tiny ones. What about the chap at the beginning, in prison in Belarus? Was he nothing to do with nothing, I mean anything, just a way of showing that Sherlock – who picks him up on all of his grammatical errors – is both heartless and a pedant? Isn't there enough going on already, without red herrings? And can we assume he was hung, I mean hanged?

I'm not happy about the London night sky, either – dark, clear, sparkly, as it is in the middle of the Atlantic maybe, but never is over the city. I'm also a bit cross with Sarah for making Watson sleep on the sofa, given that they're adults, and this is the 21st century, as we're so often reminded. Especially given what they went through last week – doing battle with a Chinese criminal gang and coming so close to death. You'd think that, after all that, some investigation of a different nature would be permitted.

And I was just a bit confused at the end, and had to get my girlfriend to explain, I'm afraid. So when camp Moriarty changes his mind and comes back to the pool to kill Holmes right there and then, he can't because Sherlock's got his gun pointing at the bomb – is that right? But if Moriarty's guys, who've all got their laser sights trained on him, shoot Sherlock quickly, surely he won't set the bomb off? Do bombs really go off if you shoot them? And what are all the flashing lights on the bombs for? Just so you know they're bombs, I suppose.

So now we enter the Great Hiatus, a terrifying void of Sherlocklessness. It was public pressure that got him back first time round, forcing Doyle to bring his great detective back from Reichenbach. It should be easier this time, given that he isn't dead, and the BBC isn't one to walk away from a success. We can still help though, so all together now: we want more, we want more . . .